The use of a linear accelerator in radiation therapy is generally known. Such linear accelerators are typically used for treating patients with x-rays or electron beams. Such x-rays are created when high energy electrons are decelerated in a target material such as tungsten. Alternatively, the electrons themselves may be used directly for treatment.
The major modules in a linear accelerator typically include a movable gantry with a treatment head, a stand, a control console and a treatment couch. The stand is typically anchored firmly to the floor and the gantry typically rotates on bearings in the stand. The operational accelerator structure, housed in the gantry, typically rotates about a horizontal axis fixed by the stand for treatment of a patient lying on the treatment couch.
In the radiation therapy treatment of a patient, geometric accuracy is a very important factor to the success of the treatment. The goal is commonly to hit a specific target, such as a tumor, and miss critical regions of the patient's body, such as the spine. Properly positioning the patient may be a critical issue in avoiding damage to tissue and critical organs. Typically, within reason, the more accurate the x-ray delivery to the exact target, the higher the dose a patient may receive.
An electronic portal image may be captured for the purpose of determining whether the target on the patient is within the treatment beam and whether critical regions of the patient are missed. Typically, people are responsible for taking these images and determining if the patient is positioned correctly. If film is used for the image, then the film must typically be developed and placed next to a reference image to compare the two images. The reference image is typically an x-ray image, which has been marked up by the patient's doctor. The two images are typically compared to ensure that the area which is actually being treated is the same area that the patient's doctor has marked up in the reference image. This comparison is typically a visual comparison. A technician may visually compare the two images and try to match visual landmarks between the two images. A potential problem with this visual comparison is human error in the comparison between the two images. The person making the comparison is commonly looking for very small errors, on the order of millimeters, which are normally very difficult to visually compare.
Another issue which may compound the problem is that high energy x-ray is commonly used. Accordingly, most of the x-ray goes through the body of the patient and a bony landmark is typically needed to give the person making the comparison an indication of the image reference. This visual comparison between a vague patient positioning image and a reference image may be substantially inaccurate.
Electronic portal imaging systems may produce an image without the use of film, however, a person still needs to visually compare the resulting image with a reference image. Some measuring tools may be used on the electronic portal imaging, however, the comparison is still substantially a manual process.
Although there are known algorithms for comparing two images electronically, there is typically no way of ensuring that the two images may be compared with the same frame of reference to ensure a proper match. The frame of reference of the portal imaging device is typically unknown due to mechanical errors. The gantry of the portal imaging device typically rotates around the patient. When the gantry is rotated, there is commonly a mechanical sag of the detector assembly in the imaging system which may shift the frame of reference of the image. Additionally, the detector housing of the imaging device is typically retractable into the gantry and the detector housing may not be exactly in the same position every time it is extended. Although the mechanical sag may be fairly slight, a millimeter or half a millimeter may still make a difference in patient positioning. Accordingly, the image may be offset compared to the reference image.
Once an image has been compared to the reference image, a multi-leaf collimator may be used to direct the treatment beam onto a selected area of the patient. However, without a well defined point of reference, the collimator may direct the treatment beam slightly off target.
Manual calibration typically requires large and unwieldy components, such as a large water tank, and a trained operator to mount and put together the various components in preparation for calibration. Calibration typically requires three to four hours. Accordingly, calibration is typically performed only about once a month with a check approximately once a week to insure that the collimator is still properly calibrated. If the check shows that the collimator is not properly calibrated, then it is typically determined whether the collimator is within an acceptable margin of error. There may be a reluctance to frequently calibrate the collimator due to the large amount of required work and time and the disruption in the scheduling of the treatments.
What is needed is a system and method for calibration that is fast and simple, without the need for additional equipment. The present invention addresses such a need.
For further background information on the construction and operation of a typical radiation therapy device, a brochure entitled "A Primer On Theory And Operation Of Linear Accelerators In Radiation Therapy", U.S. Department of Commerce, National Technical Information Service, December 1981, may be referenced.